New configuration option: notifier

The notifier, if one is set, is called whenever a song starts to play. The song title is passed as a parameter to the notifier. This can be used for system-wide song notifications.

For example, on Linux systems with [libnotify](https://wiki.archlinux.org/index.php/Desktop_notifications) installed, setting `notifier` to `notify-send` will create a desktop notification for each new song title.